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A little nuclear physics
©1996, Institut Laue-Langevin
The neutron bottle
Our understanding of the genesis of the universe
("Big Bang"theory), of physics in general and even
more down to earth problems, is based on our
knowledge of universal forces (gravitation,
electromagnetic, strong and weak electric forces) and
several fundamental physical constants. Some of these
constants are known with very high precision (speed
of light ...), whereas others are known only very
roughly and handicap the theoretical understanding of
certain phenomena observed.
Considerable effort in laboratories throughout the
world is invested to improve this knowledge. The ILL
is present in this competition with experiments such as
the neutron bottle, which has enabled the weak
interaction between quarks to be studied by measuring
the lifetime of the neutron and to define more precisely
the electric dipole moment of the neutron.
Shortly after the "Big Bang", neutrons, protons,
and electrons, the constituents of atoms (which
themselves constitute matter), were formed by
assembling even smaller particles known as quarks,
under the action of the so-called "weak electric force".
Electrons are generally considered to be eternal, as are
protons (well, almost, with a lifetime greater than 1032
years). Neutrons are however unstable, but they have
survived from the "Big Bang" until now in the nuclei
of atoms due to the forces (bonds) which hold them. A
free neutron however breaks up spontaneously after an
average lifetime of only 15 minutes (Fig. 1).
The composition of the neutron in quarks is "udd"
("u" for up and "d" for down) whereas it is "udu" for
the proton. The spontaneous disintegration of the
neutron therefore corresponds to the transformation of
a "d" quark into a "u" quark under the effect of the
weak force between quarks.
Figure 1. Spontaneous disintegration of a free neutron.
Measuring the lifetime of the free neutron is
therefore a good way of measuring the weak force
which is still not well known.
Measuring the lifetime of the neutron
The lifetime of the neutron is quite difficult to
measure with precision. in fact, today's neutron
"lamps" (nuclear reactors, spallation sources) produce
neutrons travelling at such speed that a detector one
metre long through which 2 million neutrons pass
would observe only one disintegration.
To improve on this, it would be preferable, talking
schematically, to slow down or even stop a certain
number of neutrons, put them in a bottle, and calmly
observe their disappearance over half an hour.
How can we put neutrons in a bottle?
Neutrons produced by a reactor are travelling very
fast. They pass through materials with great ease, and
it is not possible to contain them with material walls.
On the other hand a magnetic bottle has been used
successfully (storage ring for neutrons).
Other techniques have been tried throughout the
world, but that chosen by the ILL consists in creating a
flow of very slow neutrons (cold neutrons with very
low energies and very long wavelengths). These very
slow neutrons can be made to rebound indefinitely
from the walls of perfect mirrors (Figs. 2 & 3).
In fact in the same way that a prism deviates red
light more than blue light, slower neutrons are
deviated (refracted) more than fast ones by the
material through which they travel, and the slowest are
more easily reflected by a surface. U l t r a - c o l d
neutrons (the slowest) are even totally reflected by
certain surfaces, that is, once they are put into a closed
volume, a bottle, they can never get out.
extrapolated to the case for an infinitely large sized
bottle, completely free of perturbations introduced by
the container.
Ultra-cold neutrons
Figure 2. Diagram of the neutron bottle at the ILL.
This kind of bottle, made of a solid material ( glass
or metal), would unfortunately have "leaks" due to
cracks, invisible to us, but extremely large for a
particle only 10-13 cm in diameter, but also because of
a radiator effect due mainly to hydrogen atoms, always
present, which warm up or absorb neutrons with which
they collide.
The trick was to cover the walls with a special
hydrogen-free oil (no cracks and no absorption).
The last trick to obtain very precise results, was to
make a bottle whose volume could be changed. For a
given number of neutrons introduced into the bottle,
the bigger the bottle the smaller the number of
collisions between the neutrons and the walls.
This enabled the experimental results to be
How can we produce a sufficient quantity of ultracold neutrons?
The ILL reactor is the worlds' most intense source
of "thermal" neutrons. By letting them pass through a
sphere of 20 litres of liquid deuterium (an isotope of
hydrogen) at - 253 °C we obtain cold (slow) neutrons.
The slowest of these are selected by using a curved
guide (the neutron analogue of optical fibres for light).
These neutrons have an average speed of 50 m/s,
still too fast to be kept in the bottle.
They are directed towards a turbine (similar to
hydro-electric turbine), where by successive rebounds
on the blades turning at 25 m/s, their velocity is almost
cancelled (Fig. 3). They can now be put in the bottle.
The final chapter
A long series of measurements made with the
bottle in figure 2 has given a lifetime for the neutron of
887.6 ± 3 s, which combined with other results
published over the past 6 years gives a world average
of 885.9 ± 1.6 seconds.
The theoreticiens would like a precision of better
than one second. We intend to improve the neutron
bottle by taking into account the effect of gravity on
the ultra-cold neutrons.
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Figure 3. Diagram of the complete instrument.